To enhance the measurement accuracy and operability of the ultrasonic flaw detection experimental device，the flaw echo method in the pulse reflection technique and the concept of square-wave modulation waves are used. A semi-automated ultrasonic flaw detection system is built by integrating a PicoScope 6000E series oscilloscope, a fixed device for ultrasonic flaw detection, probes, etc. This system generates square-wave modulation waves through amplitude keying method. The oscilloscope then receives and analyzes the echo signals. Subsequently, Matlab performs wavelet denoising and B-scan analysis to determine the depth and specific position of the defect. The experimental results show that the relative depth error of a 10mm deep planar defect does not exceed 2.02%; The relative depth error of a 13.5mm deep circular hole defect is within 1.53%; The relative position error of a 10mm deep and 2mm wide plane defect is less than 1.49%. The improved device effectively enhances measurement accuracy and operability.